To further reduce automotive vehicle emissions, substantial effort is being made to warm up catalytic treatment devices more quickly and to control engine air/fuel ratio close to stoichiometry following a vehicle cold start. To meet certain emissions reduction goals, the exhaust stream from an engine to the catalytic treatment device must be substantially at stoichiometry or leaner within seconds following a cold start. Providing a stoichiometric mixture of fuel and air within the combustion chamber of the engine requires good fuel injection control and tightly controlled fuel characteristics. Fuel injection control precision is improving. However, mid-range volatility of commercial fuel--a critical fuel characteristic for exhaust stream air/fuel ratio control--can be highly variable depending on such factors as the geographical region, the season, and the feedstock. Accordingly, to ensure acceptable cold start driveability, the calibration used in engine air/fuel ratio control must be biased fuel rich of stoichiometry to anticipate a worst case Driveability Index (volatility) fuel. Such a calibration results in fuel rich engine operation during most conditions and use of a secondary air injection system in an vehicle exhaust system to enlean the engine exhaust to provide the required lean air-fuel stream to the catalytic treatment device. The secondary air injection system adds significantly to the cost and complexity of the vehicle. It would therefore be desirable to provide for a stoichiometric mixture of fuel and air within the combustion chamber of the engine shortly after an engine coldstart over a range of mid-range fuel volatility without the cost and complexity associated with secondary air injection systems.
It has been proposed to measure fuel volatility following a refueling operation by monitoring hydrocarbons in a fuel vapor control system over a period of time following the re-fueling operation as disclosed in copending U.S. patent application Ser. No. 08/948,501, filed Oct. 10, 1997, assigned to the assignee of this application. Engine air/fuel ratio control is then adjusted following the monitoring period to provide for improved air/fuel ratio control, including control of the air/fuel ratio of engine exhaust gasses passing to the catalytic treatment device. The shortcoming of this approach is that the air/fuel ratio in the exhaust stream to the catalytic treatment device may be difficult to precisely control during the monitoring period, which may be of significant duration.
Accordingly, it would be desirable to rapidly estimate fuel volatility following a re-fueling operation so that fuel control may be promptly adjusted in response thereto to provide for accurate control of the air/fuel ratio in the engine exhaust stream to the catalyst.